The present invention relates generally to optical systems, and more particularly, to optical digital coherent systems.
Optical signal generation method with DA (Digital to Analog) converters (DAC, in brief) has attracted a lot of interest. For example, OFDM (Orthogonal Frequency Division Multiplexing) over fiber has been proposed as one of the solutions for high-speed communication, such as 40 Gb/s or 100 Gb/s. The key part for OFDM over fiber in the transmitter side is a high-speed DAC to generate the electrical signal, before the optical modulation.
One of the implementation issues is the frequency distortion caused by the conversion speed of the electrical DAC. This is caused by the characteristic of the DAC in that its output has kind of an “output and hold” effect. This effect is illustrated in FIG. 1, where diagram (a) is the discrete signal in digital domain; and diagram (b) is the continuous signal in time domain of the digital signal output to the DAC. The DAC works like a switch: once the switch is open (triggered by its clock edge), the input digital signal is converted to a certain voltage level; after the short open period, it keeps the voltage level. This is the effect we call “output and hold”. This effect results in a sine envelope effect in the frequency domain, causing frequency distortion, as illustrated in FIG. 2, where diagram (a) is the expected output spectrum; and diagram (b) is the actual spectrum and the distortion caused by the DAC. Relying on the DAC output characteristics, another type of distortion is also possible, such as an exponential level drop.
Shown in FIG. 3 is a practical DAC sub-system. First the digital input signal 31 passes into the DAC 32. The DAC has a sine envelope frequency response similar to that illustrated in FIG. 2(b). Following the DAC, a low pass filter 33 is applied to reconstruct the signal. We can see from this system that DAC “output and hold” effect results in high-frequency distortion for the generated signal 34. This effect sometime causes unexpected waveform or noise in practical systems.
In general, an over sampling technique is used to avoid the aperture effect due to DA conversion. This is achieved by first interleaving the signals with zeroes inserted in between; then a digital low pass filter is applied to remove the high harmonic frequencies. The signal fed into the DAC is the upsampled signal. To generate very high speed modulation, there is no margin to do so in its conversion speed. So for some applications, people use pre-compensation to compensate the frequency distortion in frequency domain, before feeding the signal to the DAC. This method does not solve the problem completely, and still has some other unexpected effect.
Accordingly, there is a need for overcoming the frequency distortion caused by the conversion speed of the electrical DAC in an OFDM system.